Haaf, U.S. Pat. No. 3,639,506, discloses a blend of polyphenylene ether and a styrene resin characterized by the addition of a flame retardant combination comprising an aromatic phosphate and an aromatic halogen compound. The Haaf patent, incorporated herein by reference, discloses a wide variety of phosphates for use as the aromatic phosphate component of the blend, indicates a preference for triaryl phosphates, and states that the most preferred phosphate is triphenyl phosphate. Cooper, U.S. Pat. No. 3,883,613, incorporated herein by reference, discloses that trimesityl phosphate functions very effectively as a flame retardant per se in compositions of a polyphenylene ether and a styrene resin.
It has now been found that effective flame retardancy is provided by a new mixture of triaryl phosphates which includes at least one such phosphate wherein 1 or 2 of its 3 aryl groups are mesityl groups and its remaining group or groups are independently selected from the phenyl group and xylyl groups. It has further been found that, in resinous compositions containing a resin moiety and the mixture, the flame retardancy of the mixture per unit weight of phosphorus is greater than that of either triphenyl phosphate (TPP) per se or trimesityl phosphate (TMP) per se. Advantageously, this increased flame retardancy is obtained where the resin moieties are compositions of polyphenylene ethers and styrene resins. The latter compositions are known in the art and are described in the above-referenced Haaf and Cooper patents and in Cizek, U.S. Pat. No. 3,383,435, which is incorporated herein by reference.